1. Field of the Invention
The present invention relates to a hologram retention method, and in particular to a hologram retention method for stably retaining data recorded on a rewritable optical recording medium.
2. Description of the Prior Art
As a computer file memory of the next generation, a holographic memory having both a large capacity property resulting from a three-dimensional recording region and a high speed property resulting from a two-dimensional batch recording and reproduction scheme is attracting attention. In the holographic memory, a plurality of data pages can be multiplexed and recorded in the same volume, and the data of each page can be batch-read. By digitizing binary digital data “0, 1” as “bright, dark” instead of an analog image, and recording and reconstructing them as a hologram, recording and reproduction of digital data also become possible. Recently, proposals have been made as regards the signal-to-noise ratio, bit error rate estimation, and two-dimensional encoding based on the specific optical system and the volume multiplexing method. Studies from a more optical viewpoint, such as the influence of aberration in the optical system are also advancing.
With reference to FIGS. 5A and 5B, a shift multiplexing method, which is an example of the volume multiplexing method, will now be described. In this shift multiplexing method, a light wave having a wave surface that changes steeply, such as a spherical wave or a speckle pattern, is used as a reference beam. In the case where such a reference beam is used, the Bragg condition for reproduction can be dissatisfied by only deviating the position of the recording medium slightly from a recording spot by a shift quantity δ (FIG. 5B), and a new hologram can be recorded there. In other words, by conducting recording while slightly moving the recording medium, holograms can be multiplexed in nearly the same volume. As heretofore described, in the digital holographic storage, high speed transfer owing to the two-dimensional batch recording and reproduction and capacity increase owing to the volume recording can be implemented simultaneously.
As the medium materials for the holographic memory, photopolymer materials, photorefractive materials, azobenzen-containing polymer (azopolymer) materials, and so on are being studied extensively. Among them, in recording media using a photorefractive material or a photochromic material such as an azopolymer material, it is possible to erase recorded data and record new data. Since in these rewritable type recording media repetitive recording is possible, their use as backup memories such as hard disks besides information storage of large capacity is also greatly anticipated.
In the recording media using the photorefractive material or the photochromic material, however, there is a problem that the hologram is degraded by exposure to the light at the time of hologram reproduction. In other words, if a beam having the same wavelength as that used at the time of recording is used as the read-out beam, whenever the reproduction is conducted, diffraction efficiency of the hologram reduces, so that the bit error rate (BER) of the reproduced data increases. As a result, it becomes impossible to normally retrieve data recorded as the hologram.
In order to avoid this problem, a method of lowering the intensity of the read-out beam at the time of reproduction and reducing to the utmost the degree of hologram degradation caused by exposure to light at the time of reproduction has been proposed (Jpn. J. Appl. Phys., 34 (1995) 6439).
If the intensity of the read-out beam is lowered, however, the intensity of the reconstructed beam also falls and the BER in information readout increases. Furthermore, in general, there is no threshold in beam intensity causing photoreaction in the photochromic materials and the photorefractive materials. Even if a very weak read-out beam is used, therefore, a change occurs in recorded information. Accordingly, it is difficult to read out information from the recording medium without destroying the information. In other words, even if a very weak read-out beam is used, destruction of recorded information advances as the number of times of reading information recorded in the same volume increases.